Electromagnetic induction heating involves causing a conductive part to heat up through the circulation of currents induced by a magnetic field. This means makes it possible to heat the part as a whole without direct contact with the energy source. The part to be heated (or induced) is surrounded by at least one current circulation loop (or inductor).
The working frequencies of the generator are between a few tens of kilohertz and a few megahertz. The powers needed vary between a few kilowatts and more than a megawatt. Electromagnetic induction heating is widely used in industry and in the scientific field. In industry, it is used notably in metallurgy to refine metals, to heat treat metal parts or to produce seam-welded pipes.
Dielectric loss heating involves causing an insulating part to be heated up by provoking losses in its mass, from an alternating electrical field. The part to be heated is a mediocre insulator. It is placed between two conductive plates powered by an alternating source. A capacitor is created, the dielectric of which is the part to be heated. The generators used generally have higher working frequencies than those of electromagnetic induction heating generators. They can be between a few tens of megahertz and a few gigahertz. This heating method is used in the timber industry for drying or bonding, in the textile industry or in the manufacture or forming of plastics.
Plasma heating consists in ionizing a gaseous medium to convert it into plasma. The kinetic energy of the electrons is converted into heat. A considerable temperature rise occurs. The part to be heated is placed in the plasma. The conversion of the gaseous medium into plasma is obtained by emission from an antenna. The working frequencies of the generator are between 1 megahertz and a few tens of megahertz. The heating method is used in many industrial applications such as the fusion of refractory products, chemical synthesis, etc.
Tube power stages are used to supply the high powers needed for industrial heating, but the advances made in the field of power transistors are more and more leading to the tubes being replaced with solid-state power stages that are more flexible to use.
FIG. 1 represents a schematic diagram of an industrial high-frequency generator of the prior art including a solid-state power high-frequency amplifier 10 connected, by its power output S, to a resonant circuit 12, 12′ (amplifier load).
The power stage 10 connected to the resonant circuit 12, 12′ includes a plurality of molds M1, M2, . . . . Mi, . . . Mn with transistors coupled to the outputs of the modules by a coupling system 13 to supply the necessary high-frequency power.
The resonant circuit, with a parallel structure 12, or a series structure 12′, includes an inductor 14 to heat a part 16. An oscillator 18 (OSC) supplies a high-frequency signal, with a frequency f, to an input E of the power amplifier 10.
In the example of FIG. 1, the heating of the part 16 is produced by induction. The inductor 14 is a coil 14 coupled to the part 16. A tuning capacitor 20, 21 in parallel, or in series, depending on the structure, with the coil 14 produces the resonant circuit 12, 12′ with the induction frequency.
To obtain high HF powers (from a few kW to a few MW), the aggregation of individual transistorized RF modules M1, M2, . . . . Mi, . . . Mn, each supplying a portion of the total power as output of the heating generator, becomes the only solution in solid-state emitters.
These individual transistorized modules are voltage generators and placing voltage generators in parallel poses significant coupling problems. In practice, the voltage generators must be coupled in series (Thèvenin's principle), and the current generators have to be coupled in parallel (Norton's principle). The slightest voltage or phase offset of the voltage generators coupled in parallel would instantaneously generate very high currents in the individual modules because of their low impedance, which would immediately cause their destruction.
The parallel coupling of the individual modules, voltage generators, is all the more difficult to implement because they supply square signals. In practice, the transistors of the module operate in a block/saturated regime in order to improve their efficiency.
To avoid this parallel coupling problem because of the dispersion in the voltages at the output of the individual modules, there are known solutions.
The first solution is parallel coupling by 3 dB line coupler. This includes a load resistor referenced to ground. When an imbalance occurs between one channel and another (in phase or (and) in amplitude), the difference will be absorbed by this load, called a “bucket load”. However, the size of this type of coupler, in λ/4, is too great given the relatively low operating frequencies of the IHF emitters.
The second solution is the so-called “Wilkinson” coupler. The latter includes a floating load resistor. When an imbalance occurs between one channel and another (in phase or/and in amplitude), the difference will be absorbed by this floating load.
These two types of couplers mentioned above are widely used in the RF field for frequencies greater than 30 MHz.
The third solution is the ferrite coupler equipped with an imbalance load (or bucket load) which is a hybrid of the Wilkinson coupler. This type of coupler is used in the RF field for frequencies greater than 2 MHz.
The fourth solution is a Wheatstone bridge coupler. The latter is equipped with an imbalance load. When an imbalance occurs between one channel and other (in phase or/and in amplitude), the difference will be absorbed by this imbalance load. This type of coupler is used in the RF field for frequencies below 2 MHz, for example, to couple high-power longwave and medium wave emitters (100 kW to 1 MW).
However, these types of couplers mentioned, in addition to the cost and implementation problems, dissipate energy as pure loss in the imbalance loads or bucket loads, an energy dissipation that is associated with the output voltage differences between the coupled individual modules. This energy dissipation leads to a lowering of the efficiency of the heating generator.